Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é o tempo de fabricação com a grade 21?
 ###Tabelas: CREATE TABLE belfast_and_county_down_railway (belfast_and_county_down_railway_id AUTO_INCREMENT PRIMARY KEY, Type VARCHAR, Fleet numbers VARCHAR, Quantity made VARCHAR, Manufacturer VARCHAR, Date made VARCHAR) CREATE TABLE former_judges (former_judges_id AUTO_INCREMENT PRIMARY KEY, State VARCHAR, Born/Died VARCHAR, Active service VARCHAR, Chief Judge VARCHAR, Senior status VARCHAR, Appointed by VARCHAR, Reason for termination VARCHAR) CREATE TABLE internationally_capped_players (internationally_capped_players_id AUTO_INCREMENT PRIMARY KEY, Player VARCHAR, Country VARCHAR, Caps INT, Goals VARCHAR, Years Active VARCHAR) CREATE TABLE tallest_proposed (tallest_proposed_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Name VARCHAR, Location VARCHAR, Height feet / metres VARCHAR, Floors VARCHAR, Year Proposed VARCHAR) CREATE TABLE 250cc_classification (250cc_classification_id AUTO_INCREMENT PRIMARY KEY, Rider VARCHAR, Manufacturer VARCHAR, Laps INT, Time VARCHAR, Grid INT)  
SELECT Time FROM 250cc_classification WHERE Grid = 21